How building design changed after 9/11

One World Trade Center (center) overlooks the wedge-shaped pavilion entrance of the National September 11 Museum (lower right) and the square outlines of the memorial waterfalls in New York. Structural engineers have adopted various design changes in recent years to make buildings better able to withstand the sorts of attacks that leveled the World Trade Center’s twin towers on Sept. 11, 2001. (AP file photo by Mark Lennihan)

Shih-Ho ChaoUniversity of Texas Arlington

When a building collapses killing hundreds – or thousands – of people, there are many reasons for pause.

The loss of so much life is undoubtedly a catastrophe. Beyond that, a big building collapse raises many important engineering questions.

The collapse in 1995 of the Alfred P. Murrah Federal Building in Oklahoma City and, in 2001, of the World Trade Center towers were met with vows that nothing like those disasters would be allowed to ever happen again. For many structural engineers like me, the pledges meant not only ascertaining exactly what happened but also doing extensive research on how to improve buildings’ ability to withstand terrorist attacks.

The attack on the Murrah building taught us that a building could undergo what is called “progressive collapse,” even if only a few its columns were damaged. The building was nine stories tall and made of reinforced concrete. The explosion that brought it down, which started in a cargo truck in front of the building, weakened important parts of the building without leveling it.

Only a few columns failed because of the explosion, but as they collapsed, the undamaged columns were left holding up the building on their own. Not all of them were able to handle the additional load; about half of the building collapsed. Although a large part of the building remained standing, 268 people died in the places directly affected by the bomb, as well as in places that could no longer support themselves.

A similar phenomenon was seen in the collapse of the World Trade Center towers on Sept. 11, 2001, in which nearly 3,000 people were killed. When the buildings’ steel columns were exposed to the high temperatures created by burning airplane fuel, they became weak, causing much of the structures’ weight to be shifted onto other supports.

Until those attacks, most buildings had been built with defenses against total collapse. But the phenomenon progressive collapse was poorly understood, and rarely seen. Since 2001, we now understand the threat posed by progressive collapse. And we’ve identified two big ways to lessen both its likelihood of happening and the severity of its results if it does. We do this by improving structural design to better resist explosions and strengthening construction materials themselves.

Borrowing from earthquake protection

Research has found ways to keep columns and beams strong even when they are stressed and bent. This property is called ductility, and higher ductility could reduce the chances of progressive collapse. It’s a common concern with building in earthquake-prone areas.

In fact, building codes set by the American Society of Civil Engineers, the American Institute of Steel Construction and the American Concrete Institute have for years required structural supports to be designed with high enough ductility to withstand the sort of earthquake that’s so rare its probability of happening is once in every 2,000 years. These requirements should prevent buildings from collapsing when a massive earthquake occurs. But they’re not enough on their own to reduce or prevent damage from terrorist attacks: Underground earthquakes affect buildings very differently than nearby explosions.

Something else that engineers must consider is redundancy: That is, how to design and build multiple reinforcements for beams and columns so the loss of, say, an exterior column in an explosion doesn’t lead to the total collapse of a structure. Of the redundancy standards now in place, few can really help improve blast resistance, but the National Institute for Building Sciences does have some design guidelines.

Making concrete stronger

The materials that buildings are made of also matter. The steel columns in the World Trade Center towers lost strength rapidly when the flames from burning jet fuel on Sept. 11, 2001, reached 400 degrees Fahrenheit. Other sorts of building materials wouldn’t present the same danger.

Concrete, for instance, can be heated to that temperature without undergoing significant physical or chemical changes; it maintains most of its mechanical properties. In other words, concrete is virtually fireproof.

The new One World Trade Center building takes advantage of this. At its core are massive three-foot-thick reinforced concrete walls running the full height of the building. These walls, which are made of high-strength concrete, also contain large amounts of specially designed reinforcing bars.

In a typical explosion, large amounts of pressure are released. Concrete that’s subjected to intense stress in these conditions can be crushed.

Regular concrete can withstand 3,000 to 6,000 pounds of compression pressure per square inch; the concrete used for One World Trade Center has a compressive strength of 12,000 psi. With the use of materials science to more densely pack particles, concrete’s strength has been increased to 30,000 psi.

Improving reinforcement

Whereas the usual sort of reinforced concrete involves a framework of steel bars that’s embedded into a concrete structural element, recent years have brought further advancements. For strength and blast resistance, high-strength needle-like steel microfibers are mixed into concrete. Millions of these bond with concrete and prevent the spreading of cracks that can occur because of an explosion or other extreme force.

This mix of steel and concrete is super-strong and very ductile. Research has shown that this material, called ultra-high-performance fiber-reinforced concrete, is extremely resistant to blast damage. As a result, we can expect future designers and builders to use this material to further harden their buildings against attack. It’s just one way we are contributing to the work to prevent these sorts of tragedies from happening in the future.